Fiber optic communication is an emerging method of transmitting information from a source (transmitter) to a destination (receiver) using optical fibers as the communication channel. A Wavelength-Division Multiplexing Passive Optical Network (WDM-PON) is an optical technology for access and backhaul networks. WDM-PON uses multiple different wavelengths over a physical point-to-multipoint fiber infrastructure that contains passive optical components. The use of different wavelengths allows for traffic separation within the same physical fiber. The result is a network that provides logical point-to-point connections over a physical point-to-multipoint network topology. WDM-PON allows operators to deliver high bandwidth to multiple endpoints over long distances. A PON generally includes an optical line terminal (OLT) located at a service provider central office (e.g., a hub), a remote node connected to the central office by a feeder fiber, and a number of optical network units (ONUs) (e.g., optical network terminals (ONTs)), near end users.
An ONU typically includes a laser to generate an optical signal transmitted to the OLT. The ONU laser may be a Distributed Bragg Reflector (DBR) laser. A distributed Bragg reflector laser (DBR) is a type of stable, low-noise, single frequency laser diode. The DBR laser structure is fabricated with surface features that define a monolithic, single-mode ridge waveguide that runs the entire length of the device. The DBR laser includes a resonant cavity with a highly reflective DBR mirror on one end, and a low reflectivity cleaved exit facet on the other end. The laser emits from the exit facet opposite the DBR end. The DBR mirror reflects only a single longitudinal mode. As a result, the laser operates on a single spatial and longitudinal mode. The resonant cavity includes a gain ridge portion. The DBR is continuously tunable by changing injection current or temperature. Increasing current in the gain region causes a red shift in laser output due to heating. However, the reflectivity curve of the exit facet does not change. As a result, the exit facet will experience loss of reflectivity at the longer wavelengths and eventually will induce a discontinuous blue shift in the wavelength to find a higher gain mode.